#include <cstdio>

using namespace std;

FILE * f=fopen("dezastru.in","r");
FILE * g=fopen("dezastru.out","w");

int n,k;
double v[27],sum=0,nr=0;

inline void bk(int x,double s, int p)
{
    if(x>k)
    {
        sum += s;
        nr++;
    }
    else
        for(register int i = p+1; i <= n-k+x; ++i)
            bk(x+1,s*v[i],i);
}
int main()
{
    fscanf(f,"%d%d",&n,&k);
    for(int i=1;i<=n;i++)
        fscanf(f,"%lf",&v[i]);
    bk(1,1,0);
    fprintf(g,"%.6f",sum/nr);
    //g<<fixed<<setprecision(6)<<sum/nr;
    return 0;
}